1. Field of the Invention
This invention relates to communications systems and is particularly applicable to aircraft collision avoidance systems utilizing digital data links and associated equipment for detecting and signifying the presence of other aircraft within a predetermined range which are on a collision path with an interrogating aircraft.
2. Description of the Prior Art
Considerable effort has been applied by numerous researchers over a substantial period of time in attempts to develop a satisfactory collision avoidance system for aircraft. The only systems presently under consideration by the FAA are referred to as Traffic Alert and Collision Avoidance Systems (TCAS). They are said to surround an aircraft having such a system installed with an invisible "radar shield". While present TCAS systems have evolved from a program initiated nearly 30 years ago by the Air Transport Association of America (ATA) on behalf of its member airlines, and although the Federal Aviation Administration (FAA) has been conducting flight tests of current versions of TCAS, the FAA has yet to issue any directives indicating approval of such systems as are available.
Such recent developments have been reported in articles entitled "Preventing Midair Collisions" by Ken Julian, HIGH TECHNOLOGY, July 1985, pages 48-53, and "Enhanced Collision Avoidance System Cuts Unneeded Alerts" by Phillip J. Klass, AVIATION WEEK & SPACE TECHNOLOGY, May 20, 1985, pages 120-125. While describing some of the features of these systems, these articles also indicate some of the drawbacks and disadvantages which may perhaps account for the evident reluctance of the FAA to settle on one particular type of system and mandate its adoption by the Aviation Industry.
Among the drawbacks of the systems reported are the substantial cost of a given installation, which militates against its widespread use by General Aviation types of aircraft, and the susceptibility of the systems to overloading from saturation when substantial air traffic is present. While the articles report beneficial results in reducing the number of unnecessary alerts (false alarms), the fact that false alarms and missed alarms, at whatever low level, may be acceptable does not accord with good system design principles.
Moreover it is clear that the quest for a satisfactory collision avoidance system which is suitable for small aircraft is still underway. That the FAA's TCAS is not an acceptable answer as this nation's standard system is evidenced by the FAA's work on its replacement, called Mode-S. That Mode-S is not acceptable has been established by an assessment made by the Rand Corporation. As the results of a study made by Rand on DABS (the earlier name for Mode-S), findings were that DABS may work so long as the number of communicating aircraft does not exceed 25. The FAA provides a worst-case traffic environment as being a peak condition in the LAX basin area in 1995. Here, there may be as many as 1105 aircraft simultaneously airborne (FAA-RD-81-39). Thus, a number like 25 is woefully low. Add to this the work by the Lincoln Laboratory staff showing that surface-reflected signals by lengthy transmissions are themselves a source of destructive interference and the need for an alternative solution is clear.
Until there is a design suitable for use by all and able to provide reliability when subjected to worst-case traffic scenarios, the search will continue. This invention purports to be a solution as a means of establishing a reliable data link between multi-station nets wherein the numbers of communicating stations (aircraft) are high and the conveying media is other than wire--a solution that could well be the basis for an acceptable National Standard for an Aircraft Collision-Avoidance System.